1. Field of the Invention
This invention relates to a superconducting strand used in a transformer and the like, for permitting an AC current to flow therein.
2. Description of the Related Art
A superconducting wire for alternating current has the diameter reduced and the diameter of the wire is generally set at approx. 0.1 mm. However, if the diameter of the wire is small, the current capacity thereof becomes small, and therefore, when the superconducting wire is used for a superconducting coil, a preset number of superconducting wires are stranded together to constitute a superconducting strand.
As shown in FIG. 1, the conventional superconducting strand generally has a structure in which strands of stranded wires are stranded together, for example, six primary strands 2 each of which is constructed by stranding seven superconducting wires 1 together are stranded on the outer surface of a non-magnetic core member 3 which comprises stainless steel wires or the like. In general, the primary strand 2 is formed of 3, 7 or 19 superconducting wires of the same diameter for easy stranding.
Since heat generation due to AC loss occurs in the superconducting wire in which an AC current flows, it is necessary to sufficiently cool the superconducting wire. However, since the conventional AC superconducting strand has a structure in which strands formed of standard wires are stranded, heat removal efficiency of the superconducting wires disposed in the inner portion of the superconducting strand is lower than that of the superconducting wires disposed in the outer portion of the superconducting strand and therefore the temperature thereof tends to become high, causing a quenching phenomenon. In particular, when the superconducting strand is impregnated with epoxy resin for insulation, the above tendency becomes significant.
Further, since the conventional superconducting strand is constructed by stranding a plurality of primary strands each of which is formed of a preset number of stranded superconducting wires, the number of superconducting wires used cannot be continuously changed and it is sometimes impossible to meet the requirement for the strand of a desired current capacity.
Further, in this type of conventional superconducting strand, insulation films for the superconducting wires and core member are formed by baking insulative varnish thereon and the insulation films are generally formed to a thickness of approx. 10 .mu.m. However, with the conventional insulation films, defective insulation tends to occur between the wires because of formation of pinholes, irregularity in baking and damages given to the wires in the stranding or winding processes, and if the defective insulation has occurred and when an AC current is caused to flow in the superconducting strand, an abnormal coupling current flows between the superconducting wires, or between the superconducting wire 1 and the stainless steel wire 3 due to the difference in inductance, causing partial heat generation to break the superconducting state.